Optical communication systems are a substantial and fast growing constituent of communication networks. Wavelength division multiplexing (WDM) is one technique used to increase the capacity of optical transmission systems. A wavelength division multiplexed optical transmission system employs plural optical channels, each channel being assigned a particular channel wavelength. In a WDM system, optical channels are generated, multiplexed to form an optical signal comprised of the individual optical channels, transmitted over a waveguide, and demultiplexed such that each channel wavelength is individually routed to a designated receiver.
WDM systems have been deployed in long distance networks in a point-to-point configuration consisting of end terminals spaced from each other by one or more segments of optical fiber. In metropolitan areas, however, WDM systems having a ring or loop configuration are currently being developed. Such systems typically include a plurality of nodes located along the ring. Each node consists of one or more optical components and may include means for adding and/or dropping one or more signals. Nodes that do not allow adding or dropping of signals are typically referred to as in-line amplifiers or optical regenerators. A particular node that allows the addition and extraction of all the channels is commonly referred to as a hub or central office node, and typically has a plurality of associated add/drop elements for transmitting and receiving a corresponding plurality of channels to/from other nodes along the ring.
In a typical WDM optical communication system, information bearing optical signals corresponding to each channel is transmitted along an optical fiber by modulating light from a laser. Modulation methods are basically divided into two types: those for directly modulating a driving current of a laser using transmission data, namely, direct modulation methods, and those for modulating the output light from the laser using an external modulator such as a Mach-Zehnder interferometer, namely, external modulation methods. Although external modulation schemes effectively encode the optical signals with communication data, the external modulator is expensive and inserts additional loss into the system. Such loss, however, can be compensated in long haul networks with optical amplifiers, which further add to the cost of the system.
Shorter haul networks such as metropolitan ring networks, however, are more cost sensitive than long haul networks. Accordingly, in order to reduce the cost of these networks, semiconductor distributed feedback (DFB) directly modulated lasers have been proposed. These lasers are turned on and off directly in accordance with the communication data, thereby eliminating the need for an external modulator. For example, a commonly used prior-art direct modulation system uses the threshold current IT of a semiconductor laser as a bias current, and superimposes on the bias current a modulation current which is responsive to transmission data, thereby driving the semiconductor laser. In such modulation methods, however, the semiconductor laser must be driven by a pulse current with relatively large amplitude. This produces chirping (i.e., a dynamic wavelength shift) relative to a nominal wavelength distorting the waveform of the optical pulse propagating in dispersive optical fibers.
To decrease such chirping, a constant bias current IO, which is larger by a sufficient margin than the threshold current IT of the laser, is sometimes applied to the modulating current Im, which is typically several tens of milliamperes. Thus, modulation is performed using only that current region which is larger than the threshold current Fr of the laser. One problem with this approach is that even when a data bit of “0” is transmitted, a light emission state is maintained, thereby deteriorating the light extinction ratio of the output light.
Regardless of the particular manner in which the bias current is applied to the laser, the optical spectrum of a directly modulated semiconductor laser will be chirped in the manner shown in FIG. 1. As shown, the signal has a main intensity peak 101 at the intended channel wavelength and is spectrally broadened to include a subsidiary peak 102 at chirp-induced wavelength higher than the channel wavelength. Main peak 101 corresponds to a data bit of “1” and subsidiary peak 102 corresponds to a data bit of “0.” While the wavelength excursion between the main peak 101 and the subsidiary peak 102 can vary considerably, 0.1 nm may serve as a typical value. The wavelength excursion originating from the direct modulation of the laser includes a transient chirp term and an adiabatic chirp term. The wavelength separation between the main peak 101 and the subsidiary peak 102 depicted in FIG. 1 arises primarily from the adiabatic chirp term, which is caused by the wavelength dependency of the laser output on the modulation current that is applied to the laser for controlling the laser output's intensity.
Adiabatic, as well as transient chirp, limit the distance over which the signal can be transmitted in a dispersive fiber without experiencing an unacceptable degree of degradation. High data transmission rates as well as high-dispersion fibers such as conventional single-mode fiber exacerbate the performance degradation caused by transient chirp. While the degradation caused by the transient chirp can be reduced by well-known techniques such as dispersion compensation, the fidelity of the signal may still be deteriorated due to the adiabatic chirp. This arises when optical filters are present in the network that introduce higher loss for the main peak 101 compared to the subsidiary peak 102, which in effect reduces the fidelity of the signal.
Accordingly, it would be advantageous to provide a method and apparatus for providing a directly modulated optical signal that has a spectrally broadened adiabatic chirp that better maintains its fidelity as it propagates through an optical communication system.